J. Biol. Chem., Vol. 261, Issue 27, 12498-12505, 09, 1986
Interactions between the muscarinic receptors, sodium channels, and guanine nucleotide-binding protein(s) in rat atria
M Cohen-Armon and M Sokolovsky
The effects of the voltage-sensitive sodium channel activator batrachotoxin
(BTX) on the binding properties of muscarinic receptors were studied in
homogenates of rat atria. Also studied were the effects of muscarinic
ligands on the binding of tritium-labeled batrachotoxin ([3H]BTX) to the
same preparation. BTX (1 microM), which induces an open state in sodium
channels, enhanced the affinity of binding of several agonists to the
muscarinic receptors. Analysis of the data indicated that the effect of BTX
was to increase the affinity of the agonists toward the high-affinity
sites. Binding of antagonists was not affected by BTX. At higher
concentrations of toxin, the density of the high affinity muscarinic sites
was also affected. The binding of agonists (but not of antagonists) to
muscarinic receptors in turn enhanced the specific binding of [3H]BTX to
sodium channels. These effects on the muscarinic receptors and on the
sodium channels were inhibited in the presence of Gpp(NH)p at
concentrations lower than those bringing about conversion of binding sites
from the high affinity to the low affinity conformation. On the basis of
these findings we suggest that the opening of sodium channels and the
binding of agonists to muscarinic receptors in rat atrial membranes are
coupled events which are mediated by guanine nucleotide-binding protein(s).
Such a hypothesis is consistent with previously proposed models for signal
transduction in the membrane.